Metal components such as gears which are exposed to repetitive loads in use are required to have an increased fatigue strength on their surfaces. To meet such a requirement, shot peening has widely been employed in the art to apply compressive residual stresses by blasting the surface of the metal component with steel balls or the like.
For example, Japanese laid-open patent publication No. 5-33047 discloses a shot peening process in which blasting balls are applied to gear tooth flanks at an angle greater than if they were applied in a direction normal to the axis of the gear. The disclosed shot peening process serves the purpose of increasing compressive residual stresses of the gear tooth flanks.
While the disclosed shot peening process can increase compressive residual stresses of the tooth flanks, the blasting balls cannot sufficiently be applied to the roots of teeth because of the large angle at which the blasting balls are applied to the tooth flanks, failing to give desired compressive residual stresses to the roots or bottom lands of the gear. Furthermore, the large angle at which the blasting balls are applied to the tooth flanks makes it difficult to remove an oxide layer that has been formed on the tooth flanks by a heat treatment, and tends to leave a step between the tooth flanks and the roots of the gear.
Japanese laid-open patent publication No. 6-172850 reveals an apparatus for ejecting hard balls together with a liquid under pressure from a hard ball ejecting means to apply compressive residual stresses to the roots or bottom lands of a gear. The disclosed apparatus is capable of increasing the compressive residual stresses of the roots of the gear. However, the apparatus does not give consideration to the application of compressive residual stresses to tooth flanks.
The shot peening processes of the type described above use steel balls as the blasting balls and hard balls. Consequently, the surfaces blasted with the steel balls are roughened, making their surface roughness poor.
One solution is to employ a process of increasing the strength of a metal surface by annealing the surface of a shaped metal component, grinding the metal surface, and thereafter blasting the metal surface with glass beads whose diameters are in the range from 0.2 mm to 0.6 mm, as disclosed in Japanese patent publication No. 5-21711. This process is effective to increase the fatigue strength of the metal component without roughening the metal surface.
The above conventional process is capable of improving the metal surface roughness, but cannot increase the fatigue strength up to a desired value because of a reduction in the applied compressive residual stresses. Furthermore, inasmuch as the glass beads that are applied to the metal surface are not highly orientated, they tend to be scattered in various directions. Therefore, the blasting efficiency is very low when the glass beads are applied to the roots of a gear and gear surfaces thereof which extend from the tooth flanks to the roots of the gear.
The glass beads are crushed when they collide with the gear surfaces which are metal surfaces. Consequently, glass bead dust or particulate dust having particle sizes in microns are suspended in a processing chamber in which the metal component or gear is processed. In the processing chamber, the gear which is being processed is mounted on a spindle and rotated at a high speed, and the fine particulate dust is liable to be attached to the spindle. Therefore, the spindle tends to suffer trouble such as a rotation failure or the like.
The fine glass bead dust particles also tend to be deposited on the gear surfaces. A washing process separately provided to remove any deposited glass bead dust off the gear surfaces prevents the gear from being processed efficiently for increasing its strength.
The glass beads are usually stored in a hopper, and delivered at a given rate from the hopper through a pipe which interconnects the hopper and a nozzle from which the glass beads are ejected. Since the glass beads are small in diameter, they are apt to absorb moisture and hence clog the pipe between the hopper and the nozzle. When such a glass bead clog is developed, it is impossible to apply glass beads at a desired rate to the gear surfaces, with the result that the gear cannot stably be processed for increased strength.
It is an object of the present invention to provide a method of and an apparatus for increasing the strength of a metal component to reliably improve surface roughness and fatigue strength of the metal component with high efficiency.
Another object of the present invention is to provide a method of and an apparatus for increasing the strength of a gear to exert sufficient compressive residual stresses, produce smooth surfaces extending from the tooth flanks to the roots of the gear, and reliably remove an oxide layer from the gear surfaces.
Still another object of the present invention is to provide a method of and an apparatus for increasing the strength of a gear while efficiently and reliably removing fine glass bead dust particles.
Yet still another object of the present invention is to provide a method of and an apparatus for increasing the strength of a gear with glass beads that can be supplied accurately and reliably at a predetermined rate.